The present invention, in some embodiments thereof, relates to polymeric material science and, more particularly, but not exclusively, to HIPE-derived liquid-retaining elastomeric compositions.
High internal phase emulsions (HIPEs) are typically formed from two immiscible liquids, typically being water as a major dispersed or internal phase, and a highly hydrophobic liquid as a minor continuous or external phase, in the presence of a surfactant which is insoluble in the internal phase. The amount of surfactant needed to stabilize a major phase dispersed within a minor phase may reach up to 30% of the weight of the minor phase. HIPEs can also be stabilized through the formation of Pickering emulsions, as described below.
PolyHIPEs are highly porous polymers synthesized by polymerization of monomers within the external phase of HIPEs with internal phase volumes that are typically greater than 74% by volume of the emulsion. Most polyHIPEs are based on the co-polymerization of hydrophobic monomers and crosslinking co-monomers within the continuous phase of water-in-oil (w/o) HIPEs, followed by the removal of the internal phase, thereby producing a porous air-filled polymer.
A variety of polyHIPEs and polyHIPE-based materials have been synthesized and reported in the art. The porous morphology and properties of a polyHIPE was found to depend, among other factors, on the type and amount of the HIPE-stabilizing amphiphilic surfactant. Such surfactants are often difficult and/or costly to remove. These disadvantages become more acute for polyHIPEs where unusually large quantities of surfactant are needed, hence displacing the surfactants in HIPEs can prove advantageous, especially for polyHIPE syntheses.
High internal phase emulsions stabilized by surfactants and polyHIPEs made therefrom are disclosed, for example, in U.S. Pat. No. 6,147,131, which teaches porous polymeric materials (foams) made from HIPEs which include water-in-oil high internal phase emulsions having at least 70% of an internal aqueous phase and less than 30% of an external oil phase, wherein the oil phase comprises a vinyl polymerizable monomer and a surfactant effective to stabilize the emulsion, and wherein the surfactants are oil soluble and include an oxyalkylene component.
A Pickering emulsion (named after S.U. Pickering who first described the phenomenon in 1907) is a surfactant-free emulsion stabilized by micro- or nano-scaled solid particles that preferentially migrate to the interface between the two liquid phases. The aforementioned standard amphiphilic surfactants reduce the oil-water interfacial tension. The solid particles of a Pickering emulsion form rigid shells that surround polyhedral or spheroidal droplets of the dispersed phase and prevent coalescence thereof. The particles' shape and size, inter-particle interactions, and the wetting properties of the particles with respect to the liquid phases affect its ability to stabilize HIPEs. The stability of Pickering emulsions based on inorganic particles can be enhanced by chemically modifying the particles' surface with organic moieties that increase their tendency to migrate to the interface, and determines their ability to stabilize oil-in-water (o/w) or water-in-oil (w/o) emulsions.
Several different chemical surface modification methodologies, including silane modification, have been used to change the hydrophilic nature of the surface of silica nanoparticles such that they are able to stabilize Pickering emulsions. Silane coupling agents are commonly used to enhance fiber/matrix adhesion in polymer composites. Alkoxysilanes and chlorosilanes contain groups that bind covalently with silica through reaction with the hydroxyl groups on its surface. These silanes also contain hydrophobic organic groups that decrease surface hydrophilicity. Silane-modification thus enhances the amphiphilic character of the particles' surface, making it more suitable for Pickering emulsions and the corresponding HIPE stabilization. The extent of silica surface reaction with methyldichlorosilane was demonstrated to affect the degree of hydrophobicity and to determine whether it would stabilize an o/w or a w/o Pickering emulsion. In addition to controlling surface hydrophobicity, a silane that bears a vinyl group as part of the chemical surface modification can act as a monomer during a co-polymerization reaction.
Pickering HIPEs containing up to 92% internal phase, stabilized with 1-5% by weight of titania and silica nanoparticles, whose surfaces were modified with oleic acid, have been reported [Menner, A. et al., Chemical Communications, 2007, 4274-4276; and Ikem, V. O. et al., Angewandte Chemie International Edition, 2008, 47, 8277-8279]. Similarly, partially oxidized carbon nanotubes were used to stabilize HIPEs containing up to 60% internal phase [Menner, A. et al., Langmuir, 2007, 23, 2398-2403] and poly(methyl methacrylate) microgel particles were used to stabilize HIPEs containing 50% internal phase [Colver, P. J.; Bon, S. A. F., Chemistry of Materials, 2007, 19, 1537-1539].
Thus, the advantages of using Pickering HIPEs with a relatively small amount of nanoparticles for forming polyHIPEs, include eliminating the need for standard surfactants, eliminating the need for procedures to remove such surfactants, and eliminating the problems associated with residual and leachable surfactants. Most of the polyHIPEs synthesized from such Pickering HIPEs exhibited relatively large voids (300 to 400 μm in diameter). Smaller voids of about 50 μm in diameter were observed when poly(styrene/methyl methacrylate/acrylic acid) particles were used to stabilize Pickering HIPE [Zhang, S.; Chen, J., Chemical Communications, 2009, 2217-2219]. PolyHIPEs from Pickering HIPEs do not usually exhibit the highly interconnected porous structures typical of conventional polyHIPEs but rather exhibit a somewhat interconnected structure.
U.S. Pat. No. 6,353,037 and WO 2002/008321 teach methods for making foams which include functionalized metal oxide nanoparticles by photo- or thermo-polymerizing emulsions comprising a reactive external phase and an immiscible internal phase. Although mentioning closed-cell structures, the polymeric foams disclosed in these documents are predominantly open-celled structures, wherein most or all of the cells are in unobstructed communication with adjoining cells. “Open-celled structures” are foams wherein the majority of adjoining cells are in open communication with each other; an open-cell foam includes foams made from co-continuous emulsions in which the cell structure is not clearly defined, but there are interconnected channels creating at least one open pathway through the foam. Hence, the cells in the substantially open-celled foam structures disclosed in this document have intercellular windows that are typically large enough to permit fluid transfer from one cell to another within the foam structure. After these foams have been polymerized, the residual immiscible internal phase fluid can be removed from the foam structure by vacuum drying, freeze drying, squeeze drying, microwave drying, drying in a thermal oven, drying with infrared lights, or a combination of these techniques.
Open-cell polyHIPE structures are demonstrated and presented photographically in a study of HIPEs containing divinylbenzene and 4-vinylbenzyl chloride [Barbetta, A. et al., Chem. Commun., 2000, 221-222].
WO 2009/013500 teaches particle-stabilized high internal phase emulsions (Pickering HIPEs) comprising an internal phase, a continuous phase and particles comprising a core and a coating, wherein the wettability of the core is modulated by the coating of the particles. In the poly-Pickering-foams of WO 2009/013500, thin polymer films are formed in the area of contact points between neighboring internal-phase droplets, which rupture during the vacuum drying process and lead to a partially open porous foam structure of poly-Pickering-HIPEs. Hence, the thin polymer films which surround the droplets in the poly-Pickering-HIPEs disclosed in this document are relatively stable while the foam is wet, but as they are put under stress by the mechanical forces arising during the vacuum drying, some are forced to rupture, giving rise to some degree of interconnectivity to neighboring droplets, now pores or voids, and allows for the complete removal of the trapped internal aqueous phase.
In previous research, the present inventors investigated the synthesis of rubbery crosslinked polyacrylate materials based on Pickering HIPEs that were stabilized using silane-modified silica nanoparticles [Gurevitch, I.; Silverstein, M. S., J. Polym. Sci. A: Polym. Chem., 2010, 48, 1516-1525]. This publication describes the open-celled, interconnected porous structure and the effects of the synthesis parameters on this structure.
Additional prior art documents include U.S. Patent Application Nos. 20090215913 and 20030097103.